This invention relates to a mass spectrometer and, in particular, to a control system for use in a mass spectrometer for precisely maintaining the peak RF voltage applied to the spectrometer mass filter at a desired operating level.
Mass spectrometers are employed to measure atomic masses contained within a given substance. The spectrometer conventionally contains three basic sections including an ionization chamber, a mass filter and an ion detector. A gas or vapor of the sample to be analyzed is introduced into the ionization chamber where it is bombarded with electrons causing the sample gas to become ionized. The ions are drawn into the filter which is electrically tuned to pass only those ions having a selected charge-to-mass ratio therethrough. At the detector the selected ions are collected and a signal is generated that is indicative of the density or number of selected ions present. Usually the detector output is displayed upon a video screen or oscilloscope to provide a direct visual read out of the detected ion data.
In operation, the magnitude of the RF voltage applied to the filter is periodically changed so that ions of differing charge-to-mass ratios can be individually analyzed by the equipment. The peak RF voltage is typically changed in response to the level of a DC control signal provided by the spectrometer controller. By looking at a relatively wide spectrum of masses, the make-up of the sample can be determined. It is very important in the operation of the mass spectrometer therefore that the peak voltage produced by the RF generator be accurately and repeatably maintained over a wide dynamic operating range. In general, filter control systems used in many prior art devices fail to deliver the stability needed to meet the demand placed on today's equipment; exhibit relatively slow response times and cannot provide a linear response over a wide spectrum of atomic masses.
It should be further noted that in many prior art devices vacuum tube circuits are employed extensively to both measure the peak RF voltage applied to the filer and also to power various electrical components used in the instrument. Because of the nature of most vacuum tube circuits, the linear accuracy required of a precision mass spectrometer cannot be held over a wide operating range. Furthermore, vacuum tubes are generally energy consuming and devices do not possess the reliability of solid state components.